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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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kcmp(2) System Calls Manual kcmp(2)
kcmp - compare two processes to determine if they share a kernel
resource
Standard C library (libc, -lc)
#include <linux/kcmp.h> /* Definition of KCMP_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
int syscall(SYS_kcmp, pid_t pid1, pid_t pid2, int type,
unsigned long idx1, unsigned long idx2);
Note: glibc provides no wrapper for kcmp(), necessitating the use
of syscall(2).
The kcmp() system call can be used to check whether the two
processes identified by pid1 and pid2 share a kernel resource
such as virtual memory, file descriptors, and so on.
Permission to employ kcmp() is governed by ptrace access mode
PTRACE_MODE_READ_REALCREDS checks against both pid1 and pid2; see
ptrace(2).
The type argument specifies which resource is to be compared in
the two processes. It has one of the following values:
KCMP_FILE
Check whether a file descriptor idx1 in the process pid1
refers to the same open file description (see open(2)) as
file descriptor idx2 in the process pid2. The existence
of two file descriptors that refer to the same open file
description can occur as a result of dup(2) (and similar)
fork(2), or passing file descriptors via a domain socket
(see unix(7)).
KCMP_FILES
Check whether the processes share the same set of open
file descriptors. The arguments idx1 and idx2 are
ignored. See the discussion of the CLONE_FILES flag in
clone(2).
KCMP_FS
Check whether the processes share the same filesystem
information (i.e., file mode creation mask, working
directory, and filesystem root). The arguments idx1 and
idx2 are ignored. See the discussion of the CLONE_FS flag
in clone(2).
KCMP_IO
Check whether the processes share I/O context. The
arguments idx1 and idx2 are ignored. See the discussion
of the CLONE_IO flag in clone(2).
KCMP_SIGHAND
Check whether the processes share the same table of signal
dispositions. The arguments idx1 and idx2 are ignored.
See the discussion of the CLONE_SIGHAND flag in clone(2).
KCMP_SYSVSEM
Check whether the processes share the same list of
System V semaphore undo operations. The arguments idx1
and idx2 are ignored. See the discussion of the
CLONE_SYSVSEM flag in clone(2).
KCMP_VM
Check whether the processes share the same address space.
The arguments idx1 and idx2 are ignored. See the
discussion of the CLONE_VM flag in clone(2).
KCMP_EPOLL_TFD (since Linux 4.13)
Check whether the file descriptor idx1 of the process pid1
is present in the epoll(7) instance described by idx2 of
the process pid2. The argument idx2 is a pointer to a
structure where the target file is described. This
structure has the form:
struct kcmp_epoll_slot {
__u32 efd;
__u32 tfd;
__u64 toff;
};
Within this structure, efd is an epoll file descriptor returned
from epoll_create(2), tfd is a target file descriptor number, and
toff is a target file offset counted from zero. Several
different targets may be registered with the same file descriptor
number and setting a specific offset helps to investigate each of
them.
Note the kcmp() is not protected against false positives which
may occur if the processes are currently running. One should
stop the processes by sending SIGSTOP (see signal(7)) prior to
inspection with this system call to obtain meaningful results.
The return value of a successful call to kcmp() is simply the
result of arithmetic comparison of kernel pointers (when the
kernel compares resources, it uses their memory addresses).
The easiest way to explain is to consider an example. Suppose
that v1 and v2 are the addresses of appropriate resources, then
the return value is one of the following:
0 v1 is equal to v2; in other words, the two
processes share the resource.
1 v1 is less than v2.
2 v1 is greater than v2.
3 v1 is not equal to v2, but ordering information is
unavailable.
On error, -1 is returned, and errno is set to indicate the error.
kcmp() was designed to return values suitable for sorting. This
is particularly handy if one needs to compare a large number of
file descriptors.
EBADF type is KCMP_FILE and fd1 or fd2 is not an open file
descriptor.
EFAULT The epoll slot addressed by idx2 is outside of the user's
address space.
EINVAL type is invalid.
ENOENT The target file is not present in epoll(7) instance.
EPERM Insufficient permission to inspect process resources. The
CAP_SYS_PTRACE capability is required to inspect processes
that you do not own. Other ptrace limitations may also
apply, such as CONFIG_SECURITY_YAMA, which, when
/proc/sys/kernel/yama/ptrace_scope is 2, limits kcmp() to
child processes; see ptrace(2).
ESRCH Process pid1 or pid2 does not exist.
Linux.
Linux 3.5.
Before Linux 5.12, this system call is available only if the
kernel is configured with CONFIG_CHECKPOINT_RESTORE, since the
original purpose of the system call was for the
checkpoint/restore in user space (CRIU) feature. (The
alternative to this system call would have been to expose
suitable process information via the proc(5) filesystem; this was
deemed to be unsuitable for security reasons.) Since Linux 5.12,
this system call is also available if the kernel is configured
with CONFIG_KCMP.
See clone(2) for some background information on the shared
resources referred to on this page.
The program below uses kcmp() to test whether pairs of file
descriptors refer to the same open file description. The program
tests different cases for the file descriptor pairs, as described
in the program output. An example run of the program is as
follows:
$ ./a.out
Parent PID is 1144
Parent opened file on FD 3
PID of child of fork() is 1145
Compare duplicate FDs from different processes:
kcmp(1145, 1144, KCMP_FILE, 3, 3) ==> same
Child opened file on FD 4
Compare FDs from distinct open()s in same process:
kcmp(1145, 1145, KCMP_FILE, 3, 4) ==> different
Child duplicated FD 3 to create FD 5
Compare duplicated FDs in same process:
kcmp(1145, 1145, KCMP_FILE, 3, 5) ==> same
Program source
#define _GNU_SOURCE
#include <err.h>
#include <fcntl.h>
#include <linux/kcmp.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
static int
kcmp(pid_t pid1, pid_t pid2, int type,
unsigned long idx1, unsigned long idx2)
{
return syscall(SYS_kcmp, pid1, pid2, type, idx1, idx2);
}
static void
test_kcmp(char *msg, pid_t pid1, pid_t pid2, int fd_a, int fd_b)
{
printf("\t%s\n", msg);
printf("\t\tkcmp(%jd, %jd, KCMP_FILE, %d, %d) ==> %s\n",
(intmax_t) pid1, (intmax_t) pid2, fd_a, fd_b,
(kcmp(pid1, pid2, KCMP_FILE, fd_a, fd_b) == 0) ?
"same" : "different");
}
int
main(void)
{
int fd1, fd2, fd3;
static const char pathname[] = "/tmp/kcmp.test";
fd1 = open(pathname, O_CREAT | O_RDWR, 0600);
if (fd1 == -1)
err(EXIT_FAILURE, "open");
printf("Parent PID is %jd\n", (intmax_t) getpid());
printf("Parent opened file on FD %d\n\n", fd1);
switch (fork()) {
case -1:
err(EXIT_FAILURE, "fork");
case 0:
printf("PID of child of fork() is %jd\n", (intmax_t) getpid());
test_kcmp("Compare duplicate FDs from different processes:",
getpid(), getppid(), fd1, fd1);
fd2 = open(pathname, O_CREAT | O_RDWR, 0600);
if (fd2 == -1)
err(EXIT_FAILURE, "open");
printf("Child opened file on FD %d\n", fd2);
test_kcmp("Compare FDs from distinct open()s in same process:",
getpid(), getpid(), fd1, fd2);
fd3 = dup(fd1);
if (fd3 == -1)
err(EXIT_FAILURE, "dup");
printf("Child duplicated FD %d to create FD %d\n", fd1, fd3);
test_kcmp("Compare duplicated FDs in same process:",
getpid(), getpid(), fd1, fd3);
break;
default:
wait(NULL);
}
exit(EXIT_SUCCESS);
}
clone(2), unshare(2)
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2024-06-26. If you discover any rendering problems in this HTML
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part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.9.1 2024-06-15 kcmp(2)
Pages that refer to this page: clone(2), open(2), pidfd_getfd(2), ptrace(2), syscalls(2), unshare(2), capabilities(7), epoll(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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